| Molybdenum and its alloys have excellent high temperature mechanical properties,good electrical conductivity,thermal conductivity and corrosion resistance,and are widely used in aerospace,nuclear industry and electronics.However,molybdenum and molybdenum alloys are easily oxidized under high temperature and aerobic conditions,forming volatile oxides,resulting in a sharp drop of their high temperature mechanical properties,which limits their high temperature applications.Molybdenum exhibits the low degree of alloying,so in order to improve the high temperature oxidation resistance of molybdenum and its alloys,the most effective method is to coat a high temperature oxidation resistant coating.In the high temperature and aerobic environment,the intermetallic compound MoSi2 can form a continuous SiO2 protective film on the surface,which can effectively prevent the diffusion of oxygen to the substrate,thus providing good high temperature protection for molybdenum and its alloys.In this study,MoSi2 coating was prepared on the molybdenum surface by solid pack cementation.The effect of different activators to the microstructure of the coating was studied,and the thermodynamics kinetics of the coating formation were analyzed.The surface and cross-section structure of the coating were analyzed by X-ray diffraction,scanning electron microscope and energy dispersive spectroscopy.It is found that MoSi2 coating is formed on the molybdenum surface after pack cementation,and the inner layer of the coating mainly consists of MoSi2 and Mo5Si3.Through comparative,it was found that use of NH4F as an activator to prepare MoSi2 coating is more efficient and thicker than the use of NaF.The oxidation properties of the coatings at low temperature(600?)and high temperature(1400?)were tested.It is found that low temperature oxidation resistance(600?)of the coating was poor and high temperature oxidation resistance(1400?)was good.However,at 1400? the coatings was degraded and the service life declined.In order to improve the oxidation resistance of the coating,B-Si co-deposition and two-step pack cementation were used to prepare B-modified MoSi2 coating.MoB was the main internal layer and MoSi2 was the main external layer.There were two-phase mixing zone in the co-deposition coating,and a lot of holes in this area due to the precipitation of MoB.The oxidation resistance of the two coatings at low temperature(600?)and high temperature(1400?)were tested respectively.It was found that the addition of B element improved the oxidation resistance of the coatings at low temperature,and the improving effect of the two-step pack cementation was more obvious.In two coatings,Mo5SiB2 phase is formed when Si diffuses to MoB layer at 1400?and Mo5SiB2 can birrier the diffusion of Si and slow down the degradation of the coating,and the effect of the coating prepared by two-step is more obvious.So,the B-modified MoSi2 prepared by two-step pack cementation not only improve the oxidation resistance at low temperature,but also enhances the service life of the coating at high temperature.In addition,in order to improve the stability of the oxide scale on the surface of the coatings,Cr modified MoSi2 coatings were prepared by Si-Cr co-infiltration.After oxidation at 1400?,a composite oxide film of SiO2 and a small amount of Cr5O12 appears on the surface of the coating,and the stability of the oxide film is improved.However,the MoSi2 inside the coating has completely changed to Mo5Si3,and the coating is completely degraded.Therefore,the two-step pack cementation,which is the Si-Cr co-deposition after boronisation,is used to prolong the service life of the coating.After deposition,the outside of the coating is still MoSi2 phase,and the inner layer of the coating is MoB phase.After oxidation,similar with the coating prepared by Si-Cr deposition,the surface of the coating forms a composite oxide,and a Mo5SiB2 phase is formed in the coating,and the MoSi2 layer still exists,indicating that the diffusion of Si is effectively slowed down,and the service life of the coating increased.So,the B-Cr modified MoSi2 coating not only has good high temperature stability,but also increases the service life,it provide better high temperature thermal protection for molybdenum and its alloys. |
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